1. Field of the Invention
The present invention relates to a method for selecting routes in a communications network. This invention is particularly suitable for selecting routes in a wireless network formed by a plurality of wireless nodes.
2. Description of the Related Art
In a communications network where wireless cell-sites are interconnected by wireless links (i.e., radio beams) to form a wireless access network and the latter is connected to a wireline core network via optical links, each cell-site functions as a source/destination node for a mobile terminal or a transit node for repeating packets between neighbor cell-sites. Since communication within the access network is predominantly a flow of packets to and from the core network, packets from the access network follow the pattern of a tree, with a node on the core network serving as a root of the tree topology.
Two routing algorithms are available. One is the distance vector routing algorithm, the other being the link state routing algorithm. It is known that distance vector routing is suitable for a tree topology. Further, distance vector routing is preferred over link state routing because the former requires less routing information than the latter. In distance vector routing, a tree topology wireless network is built up by initially broadcasting a control packet from a core (root) node, containing its node identifier in its source address field and a metric field which is set to zero. An access node that receives it reads the source node identifier and updates the metric value by adding to it the cost of the route on its upstream side to the source node. If the hop count number is used as a measure of the route cost the metric is updated by adding the hop count “1” to the received metric value. If the transmission loss is used as a measure of the route cost, the access node calculates the transmission loss of the received RF signal from its field strength and sums the calculated value to the received metric value. The access node then stores the updated metric in its memory. Initially, there is only one metric value that is stored in the memory. Hence the source node is recognized as an upstream node. Then, the access node broadcasts a control packet containing the updated metric, with its source and destination address fields being set with its own identifier and the identifier of the upstream node, respectively. As the same process is repeated on neighbor access nodes, a sufficient amount of metric data is stored in their memory to select a least-cost route to the core node.
A wireless network configured in his way is characteristic of the metric used. If the hop count is used as a metric value, the network will be comprised of least-hop-count routes. If the transmission loss or signal-to-interference-noise ratio is used, the network will ensure high quality transmission. On the other hand, the use of hop counts will result in a network where the nodes are spaced at long distance apart. Hence, signals suffer distortion and interference. Further, the use of transmission loss and SINR as a metric will result in a network dominated by high number hop-count routes where signals experience long processing delays. Thus, the characteristic of a resulting network not only has the advantage of the metric used but also has its disadvantage.